Moving forward, gravelling the vacuum is the least of your problems with that stock.
I'm curious, was this purchase one of those deals you see now and again where someone's selling a tank but you have to take the fish that are already in it too, so basically you've been lumbered with a poor stocking choice from the previous owner?
.....Or, did you buy this tank brand new and the stocking is completely your own doing?
Either way, it seems to me that research wasn't high on the agenda.
You don't have a lot of time before things become unmanageable, you need to make some swift decisions.
Personally, I'd get rid of the lot and just keep a solo oscar. Yes, that's how bad your current stocking is.